Prostate cancer (PCa) depends on androgenic signaling for growth and survival. Androgens exert their cellular and physiologic effects through binding to the androgen receptor (AR), a member of the steroid hormone receptor family of transcription factors. The human AR gene is located on chromosome Xq11-12 and spans approximately 180 kb of DNA with eight known exons. The prototype AR protein contains several functional domains. The NH2-terminal domain (NTD), encoded by exon 1, constitutes approximately 60% of the 110-kDa full-length protein and is the transcriptional regulatory region of the protein. The central DNA-binding domain (DBD) is encoded by exons 2 and 3, whereas exons 4 to 8 code for the COOH-terminal ligand-binding domain (LBD). Androgen binding to the AR LBD allows entry of the ligand-bound receptor into the nucleus and subsequent transcriptional regulation of androgen-responsive genes.
Hormonal therapy has been used since 1941 for the treatment of metastatic prostate cancer. Hormone deprivation therapies employing surgical and/or medical castration as well as their combination with anti-androgens have since become the mainstay of systemic treatment for advanced prostate cancer. Hormonal therapies for advanced PCa target AR-mediated functions by suppressing the production of androgens and/or androgen binding to the AR LBD. Although these therapies often result in a period of clinical regression, they are not curative due to progression to hormone-refractory PCa (HRPC) for which effective therapeutic options are limited. In a contemporary clinical setting, the length of clinical remission, often assessed by serum prostate-specific antigen (PSA) measurements, varies substantially due to a wide spectrum of clinical phenotypes among treated patients. Almost invariably, however, prostate cancer develops castration-resistant phenotype and progresses to a life-threatening stage, despite hormone therapies. The widespread use of hormone deprivation therapies is manifested in the observation that almost all patients who die from prostate cancer had received and failed hormone-deprivation therapies.
A few lines of evidence have established that, unlike human breast cancer, prostate cancer progression upon hormone therapy is not due to loss of dependence on hormonal signaling but, instead, characterized by sustained androgenic signaling that bypasses the requirement for physiological levels of androgens. First, with only certain exceptions, prostate cancer patients dying from castration-resistant prostate cancer have very high levels of serum PSA, the production of which is driven by androgenic signaling. Second, castration-resistant prostate cancers have elevated expression levels of the key mediator of androgenic signaling, the AR, and this is a very consistent molecular feature in tissues derived from patients with castration-resistant prostate cancer. Third, a subset of prostate cancers that relapsed following hormone therapy continue to respond to second-line hormone therapies designed to disrupt the AR signaling axis, suggesting that AR-mediated androgenic signaling is still operating among these tumors. While it is possible that AR-negative prostate cancer cells may give rise to androgen-independent prostate carcinoma, prostate tumors comprised of mainly AR-negative malignant cells (i.e., small cells and neuroendocrine cells) are rare.
AR-mediated functions are not completely abrogated by the existing hormone therapies. HRPC continues to depend on AR-mediated functions but bypasses the requirement for physiologic levels of androgens. Molecular alterations involving AR itself, such as AR overexpression and gain-of-function AR LBD mutations, are common in HRPC and allow for continued AR-mediated genomic functions under the presence of reduced or altered ligands. Despite the established clinical relevance of these well-characterized AR alterations in HRPC, only a few previous studies have suggested an alternative mechanism for HRPC and investigated the putative role of AR variants lacking the AR LBD.
Accordingly, a need remains to for more effective compositions and methods for the treatment of prostate cancer.